1. Field of the Invention
The present invention relates to a compression-ignition type engine.
2. Description of the Related Art
In a compression-ignition type engine, the degree of dispersion of the fuel injected into the combustion chamber has a major effect on the combustion. That is, if the fuel is made to disperse throughout the combustion chamber, the amount of heat generated per unit volume becomes lower, so the combustion temperature becomes lower and therefore smooth combustion is achieved without the generation of NOx. Further, since there is sufficient air present around the fuel particles, soot is no longer generated either. Therefore, known in the art is a compression-ignition type engine which is designed to inject fuel during the compression stroke before 60 degrees before top dead center so as to cause the injected fuel to disperse throughout the combustion chamber (see Japanese Unexamined Patent Publication (Kokai) No. 7-317588).
That is, if the pressure in the combustion chamber becomes high, the air resistance becomes larger, so the injected fuel has difficulty spreading throughout the entire combustion chamber. Therefore, this compression-ignition type engine was designed to inject the fuel before 60 degrees before top dead center in the compression stroke where the pressure in the combustion chamber is low.
Further, known in the art is a diesel engine designed to inject a large amount of fuel into the combustion chamber early on to form a lean air-fuel mixture and then to inject a small amount of fuel and cause the injected fuel to ignite when the engine is operating under a low load and to inject a small amount of fuel into the combustion chamber early on and then inject a large amount of fuel and cause the injected fuel to ignite when the engine is operating under a high load (Japanese Unexamined Patent Publication (Kokai) No. 8-82219).
When causing the injected fuel to disperse throughout the combustion chamber as in the compression-ignition type engine described in Japanese Unexamined Patent Publication (Kokai) No. 7-317588, gentle combustion is achieved without the generation of NOx and hydrocarbons if the amount of injected fuel is small. When the amount of fuel injected becomes larger, however, even if trying to cause the injected fuel to disperse throughout the combustion chamber, the fuel starts igniting early. Once the fuel ignites early, the temperature in the combustion chamber rises, so the fuel ignites even earlier. As a result, the combustion gradually becomes more intense and not only will knocking occur, but also large amounts of NOx and soot will be produced.
In this way, in a compression-ignition type engine, the ignition timing can no longer be controlled to the ignition timing giving a gentle combustion when the amount of injected fuel becomes large. If it were possible to control the ignition timing to the ignition timing giving gentle combustion in this case, then it would be possible to achieve gentle combustion with little generation of NOx and soot.
Further, the ignition timing is not controlled in the diesel engine described in Japanese Unexamined Patent Publication (Kokai) No. 8-82219 either.